1. Field of the Invention
The present invention relates to a printing apparatus. More particularly, the invention relates to a printing apparatus employing an electrothermal transfer film which can be heated upon electric conduction therethrough. Specifically, the invention relates to a so-called electrothermal printing apparatus for printing characters, figures, or other patterns by using an electrothermal transfer film composed of a resistive layer and an ink layer, by applying a printing signal so as to pass a current higher than a predetermined level through a selected portion of the resistive layer in the film to fuse the thermally fusible ink by Joule heat, and by transferring the fused ink to a recording body such as a paper sheet, a film or the like.
2. Description of the Prior Art
One known electrothermal printing apparatus is disclosed in U.S. Pat. No. 4,350,449 wherein the disclosed printing device effects printing in response to a printing signal applied when a predetermined current is passed through a resistive layer in an ink film. With this device, if electrodes in the print head are not held sufficiently in contact with the resistive layer, then small dots cannot be printed, and dots of different diameters will be printed where larger dot diameters are desired.
Another known apparatus is disclosed in U.S. Pat. No. 4,236,834. The device of this patent uses head electrodes which are constructed as projecting needles, necessitating the use of large inter-electrode pitches which result in an inability to print dots at small pitches. Further, since current passes through each electrode, those electrodes which are better conductors of current wear more rapidly than other electrodes. Therefore, the lengths of the electrodes become irregular during use, with the consequence that the print head electrodes contact the recording body unevenly. The various problems referred to above have made it difficult to put the above-described thermal transfer printing apparatus to practical use.